Acoustic device

An acoustic device, e.g. a loudspeaker, comprising a resonant multi-mode acoustic radiator panel having opposed faces, a vibration exciter arranged to apply bending wave vibration to the resonant panel to produce an acoustic output, a cavity enclosing at least a portion of one panel face and arranged to contain acoustic radiation from the portion of the panel face, wherein the cavity is such as to modify the modal behaviour of the panel.Also disclosed is a method of modifying the modal behaviour of a resonant panel acoustic device, comprising bringing the resonant panel into close proximity with a boundary surface to define a resonant cavity therebetween.